Steel that is used in, for example, automobile parts and construction materials, is a metal apt to rust. Zinc plating and alloy plating mainly using zinc have been widely used for a long time as a method for protecting such metals apt to rust from corroding. In particular, among zinc alloy plating, zinc-nickel alloy plating has been increasingly widely used for automobile parts because of its excellent corrosion resistance. In a specific method of this zinc-nickel alloy plating, a plating solution dissolving a compound of zinc and nickel in a weak acid or alkali aqueous solution is subjected to direct current electrolysis to deposit the alloy on the cathode.
Zinc-nickel alloy plating has been applied to mass production parts for several decades. In the early period of such a history, mostly used was a bath providing a proportion of nickel in plating film of about 6% to 10% by mass (hereinafter, referred to as low-nickel-bath). Thereafter, a bath having a proportion of nickel of 11% to 19% by mass, more typically 12% to 18% by mass, (hereinafter, referred to as high-nickel-bath) has been developed. Application of this high-nickel-bath has been increasing because of its further excellent corrosion resistance.
Most of the high-nickel-baths that are currently practically used are alkali baths, and acid baths are rarely used. The reason thereof is, for example, that an alkali high-nickel-bath can stably provide plating films with the above-mentioned range of proportion of nickel and can achieve high adhesion to plating films. Accordingly, alkali high-nickel-baths are employed for, for example, automobile parts.
As an example of zinc plating using such an alkali high-nickel-bath, Patent Literature 1 discloses a plating solution containing an amine having four or more nitrogen atoms in one molecule.
However, the alkali high-nickel-bath has some disadvantages. For example, the alkali high-nickel-bath has a low current efficiency in plating and has a low plating rate. In addition, long time use thereof increases the carbonate content in the plating solution to further decrease the current efficiency, and the amount of nickel in the plating film becomes too high, exceeding the above-mentioned range, to lose the sacrificial rust resistant effect on iron materials. As a result, the life-span of the plating solution is restricted. In addition, a ratio of nickel higher than the above-mentioned range in a plating film deteriorates the adhesion of the plating.
In contrast, a weak acid bath gives a high current efficiency and has a high plating rate. In addition, accumulation of carbonate does not occur, unlike alkali baths. For example, Patent Literature 2 discloses a zinc-nickel trialloy plating solution containing an amine compound.